Extended linear power amplifier systems have been proposed for use in cellular base stations. These systems include a number of linear power amplifier modules producing a corresponding number of output signals that need to be combined into a single high power signal before transmission. Such systems are typically used in communication systems such as in a base site of a radiotelephone system. In such systems, it is desirable that the number of linear power amplifier modules may be any number from a minimum number up to a maximum number of allowable amplifiers. In this manner, the amount of power output by the transmission unit of the base station may be adjusted by selecting the number of amplifiers to be used. In addition, it is desirable that the system provides adequate power amplification efficiency across the entire range of selected amplifiers, i.e. from the minimum number to the maximum number of amplifiers and that dissipations in the amplifiers due to reflected output power be kept to a minimum, particularly when the system contains a maximum of amplifiers.
However, conventional power combiners, such as Wilkinson type combiners, are only efficient when all of the available amplifiers are present, and rapidly become less efficient due to impedance mismatch and resistive circuit elements when less than the maximum number of amplifiers are selected. For example, a Wilkinson type 10:1 combiner will only be 10% efficient when a single amplifier is coupled to the combiner. This poor efficiency at less than maximum amplification is undesirable for extended linear amplifier applications A proposed device for addressing this problem is described in U.S. Pat. No. 5,543,751, by Stedman et al., and incorporated by reference herein. Stedman et al. describes a power combiner having matching transmission lines of substantially equivalent length of one-quarter of a wavelength, and a characteristic impedance determined according to a function of the minimum and the maximum number of amplifiers in the set of amplifiers. Although the Stedman et al. patent does solve the above-mentioned problem, the solution results in a high level of reflected power to the amplifiers particularly when the maximum number of unequal gain amplifiers are used. This condition results in undesirable heating in the power amplifiers.
Accordingly, there is a need for an improved power combiner that maintains a high efficiency across the entire range of amplifiers having differing gain values, that may be coupled to the combiner without degrading the useful bandwidth of the combiner and minimizes heating in the power amplifiers due to reflection created by the combining a maximum quantity of unequal gain amplifiers.